Touch panels having icons that can be pressed to perform a function such as switching on a light or fan are becoming more desirable in domestic, building, and automotive applications, such as household appliances, vehicle heating controls, and the like. Typically, the touch panels have a relatively uncluttered surface with one or more icons (that may or may not be back lit) visible on the surface at all times or visible when illuminated. In use, a user is able to touch the icon to activate an electronic switch, which then performs a function. These touch panels have advantages over traditional panels featuring mechanical switches through the provision of a less cluttered visual appearance.
Two common technologies used for touch panels are surface capacitance and projected capacitance. Surface capacitance works on the basis of a person creating a path for an electrical current to ground. When the touch screen is touched, current will flow through the person's finger to ground. One side of an insulator is coated with a conductive layer. A small voltage is applied to the layer resulting in a uniform electrostatic field. A sensor can determine the location of the touch directly from the change in capacitance measured from the four corners of the panel. The greater the change in any corner, the closer the touch is to that corner. However, surface capacitance technology provides limited positional resolution, requires calibration during manufacture, does not work if a user is wearing gloves and is difficult to implement on geometrically complex parts.
Projected capacitance panels are frequently used on tablets and smartphones and incorporate a grid of transparent conductive material, such as indium tin oxide (ITO), organised into rows and columns. A voltage applied to the grid creates a uniform electrostatic field and when a conductive object, such as a finger, comes into contact with the panel, it increases the capacitive load at that point. Capacitive panels are normally transparent and to date it has not been possible to use them to provide an aesthetically pleasing continuous metallic surface with touch functionality in an appliance or vehicle interior trim component. This is because the continuous metallic surface capacitively couples to the transparent conductive grid and the resultant large parasitic capacitance makes it very difficult to detect a small capacitance increase due to the touch of a user.
It would be beneficial to provide an aesthetically pleasing touch panel that appears as a continuous metal surface whilst providing hidden until lit icons and discrete switching capability associated with the icons.